1. Technical Field
The present disclosure relates generally to downhole electromagnetic logging and, more particularly, to methods for selecting bed boundaries and log squaring tec
2. Background Information
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic (resistivity) tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties are also available.
Early logging tools were run into a wellbore on a wireline cable after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, as the demand for information while drilling a borehole continued to increase, measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools have since been developed. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, NMR distributions, and so forth. MWD and LWD tools often have characteristics common to wireline tools (e.g., transmitting and receiving antennas, sensors, etc.), but are designed and constructed to endure and operate in the harsh environment of drilling.
The use of electromagnetic measurements in downhole applications, such as logging while drilling (LWD) and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with other formation measurements (such as porosity), can be used to indicate the presence of hydrocarbons in the formation. Moreover, azimuthally sensitive directional resistivity measurements are commonly employed, i.e., in pay-zone geo-steering applications, to provide information upon which steering decisions may be made, for example, including distance and direction to a remote bed. Directional resistivity tools often make use of tilted or transverse antennas (antennas that have a magnetic dipole that is tilted or transverse with respect to the tool axis). Non-directional tools often refer to those that use antennas having magnetic dipoles that are parallel with the tool axis. Further, some electromagnetic logging tools may be capable of making both directional and non-directional measurements.
As can appreciated, once a target area or pay-zone has been identified (e.g., from seismic interpretation, geological mapping, and/or petrophysical analysis) all well trajectory can be planned. Accurate well placement has been a challenge for the industry, and is complicated by a number of factors, such as uncertainty in a target's position and unpredictable structural and/or stratigraphic variations. Particularly, in more recent years, the demand for accurate well placement to produce high angle and/or horizontal wells (sometimes referred to as HAHZ) with complex steering trajectories in subterranean formations has increased, as often these types of wells can at times be more efficient at draining reservoirs and recovering hydrocarbons when compared to the more conventional non-steered wells.
Building layered earth models based formation measurements can be useful in helping drillers make steering decisions. For example, both non-directional and/or directional electromagnetic resistivity measurements have been used to assist with constructing layered models. This process, referred to as “log squaring,” uses electromagnetic measurement logs to determine the location of bed boundaries and to assign assumed formation properties to each layer, in this case horizontal resistivity (Rh) and vertical resistivity (Rv). However, depending on the well angle, either non-directional or directional measurements can be more suitable for defining bed boundaries. It would be desirable to have a technique for log squaring that can provide reliable information in all wells regardless of the well's inclination angle.